unit tests f-test and exponential distribution #225 , removing wilcoxon distribution #213

[zieglerSe]

Added unit tests for following topics (with edgecases): F-test:

• regular F-test
• NaN
• Infinities
• regular F-test with ties

exponential Distribution:

• fix of formula
• parameter check
• lambda regular.NaN, infinity
• score regular, NaN, infinity
• Mean, Standard Deviation and Variance with regular & NaN

wilcoxon distribution:

• removed, because it was obsolete

[bvenn]

The testing dataset added to tests/data is not used in your tests. Is this a work-in-progress-PR and unit tests for SAM are on their way or do you want to merge this PR and add SAM-tests afterwards? For future commits try avoiding merge commits by either rebasing or fetching the latest changes before committing. There are some merge conflicts due to prior changes in distribution modules. I'll take care of it if the PR is ready to merge :+1:

[codecov-commenter]

Codecov Report

Merging #226 (7bfd855) into developer (d0fbe24) will increase coverage by 0.57%. The diff coverage is 92.18%.

@@              Coverage Diff              @@
##           developer     #226      +/-   ##
=============================================
+ Coverage      40.46%   41.03%   +0.57%     
=============================================
  Files            125      125              
  Lines          12990    13017      +27     
  Branches        1787     1779       -8     
=============================================
+ Hits            5256     5342      +86     
+ Misses          7287     7224      -63     
- Partials         447      451       +4     

Impacted Files	Coverage Δ
src/FSharp.Stats/Distributions/Discrete.fs	20.76% <ø> (+4.50%)	:arrow_up:
tests/FSharp.Stats.Tests/Main.fs	0.00% <0.00%> (ø)
src/FSharp.Stats/Distributions/Continuous.fs	24.57% <100.00%> (+3.43%)	:arrow_up:
tests/FSharp.Stats.Tests/Distributions.fs	98.43% <100.00%> (+0.17%)	:arrow_up:
tests/FSharp.Stats.Tests/Testing.fs	100.00% <100.00%> (ø)
src/FSharp.Stats/SpecialFunctions/Beta.fs	69.04% <0.00%> (+2.38%)	:arrow_up:
src/FSharp.Stats/Testing/TestStatistics.fs	85.71% <0.00%> (+28.57%)	:arrow_up:
src/FSharp.Stats/Testing/FTest.fs	56.25% <0.00%> (+56.25%)	:arrow_up:

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